Circuit Description - ViewSonic VA1930wm-1 Service Manual

4. Circuit Description

2. Electronic Circuit Theory
2.1 Switching Mode Power Supply
2.1.1 AC Current Input Circuit
P801 is a connector for connecting AC Power. F801 is a fuse to protect all the circuit. AC
input voltage is from 90v to 264V. R801 and R802 joined between two inputting main circuit to
prevent man from shock. L801 is used to clear up low frequency wave. C801 and C802 are used to
discharge the waves that L801 produced. High frequency waves are damped by C801 and C802.
D801 is a rectifier which composed of 4 build-in diodes, it inverts AC to DC.
2.1.2 High Voltage to Low Voltage Control Circuit
C804 is used to smooth the wave from rectifier. IC802 is a highly integrated PWM controller.
When rectified DC high voltage is applied to the HV pin during start-up, the MOSFET Q804 is
initially off, and the Vcc pin capacitor is charged. When the Vcc pin voltage reaches approximately
10V, the control circuitry is activated and the soft-start begins. The soft-start circuit gradually
increases the duty cycle of the MOSFET from zero to the maximum value over approximately 4ms.
If no external feedback/supply current is fed into the FB pin by the end of the soft-start, the current
Setpoint will be above the fault level, FAULT flag is raised, if the FAULT duration exceeds 80ms,
the output controller disable
Resistor R808, R809, R810, R811 are for line over voltage shutdown(OVP)
When PWM is turned off, the main current flow will be consumed through R804 and D802,
This will prevent MOSFET Q804 from being damaged under large current impulse and voltage
spike.
D803 and C807 to provide internal Auxiliary voltage to Vcc pin during normal operation.
Otherwise, error amplifier and feedback current input the FB pin for duty cycle control.
2.1.3 DC_5V and DC_14V Output Circuit
For DC 5V, D805 is used to rectify the inducted current. R828 and C814 are used to store
energy when current is reversed. The parts including C818, C822, C820,L803 are used to smooth
the current waves.
For DC 14V, D803 is used to rectify the inducted current. R827 and C813 are used to store
energy when current is reversed. The parts including C815, C817 and L802 are used to smooth the
current waves.
2.1.4 Feedback and OVP Protect Circuit
Pin R of IC803 is supplied 2.5-v stable voltage. It connects to 5V and 14V output through
R822, R823 and R824. R822, R823 and R824 are output sampling resistor. When the sampling
voltage more than 2.5V or less than 2.5V, current of FB IC802 will change, this can change the
voltage from T801.
OVP Protect Circuit: When output is overvoltage, the auxiliary winding voltage will be
increased, when it reaches about 14V. Q803 is triggered . It makes the IC802 Pin 1 exceed 5V, then
the IC802 output will be disabled.
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VA1930wm-1